Methods, in which the geographical position of a vehicle is determined with the help of the signals of navigation satellites, in particular of the GPS satellites, are generally known and form the basis of modern vehicle navigation systems. The positioning accuracies that are conventionally achievable with these signals of the order of magnitude of a few meters are adequate in order to correctly identify in the very most cases a road, on which a vehicle using the navigation system is located, and in order to be able to issue navigation instructions to the driver of the vehicle on that basis. For further automation applications, however, in particular for fully automatic travelling, the accuracy of the popular methods is not yet adequate.
A reason for inaccuracies during the GPS positioning are fluctuating signal propagation delays between the transmitting satellite and the receiving vehicle due to atmospheric interferences. A further cause of measurement inaccuracies can be an unfavorable position of a satellite in which the propagation delay of its signal to the vehicle does not adequately vary with its geographical position or propagation delay distortions, which can occur in particular in heavily built-up cities, when the direct visual connection between satellite and vehicle is blocked by a building and the signal of the satellite cannot reach the vehicle by direct route, but only after a reflection.
In order to improve the positioning accuracy it has been proposed to install stationary receiver units along the roads which in turn receive the satellite signals, carry out an estimate of their own position by way of the satellite signals and calculate the deviation between this estimated and a true position, passing it on to the passing vehicles. When the causes of errors, to which the stationary station and the passing vehicle are exposed are identical, then the error between estimated and true position for vehicle and stationary station should also be the same and a true position of the vehicle can be determined by vectorially adding the deviation that is determined by the stationary station to the estimated position of the vehicle.
The problem with this approach however lies in that the causes of errors in turn are position-dependent and consequently the deviation between the position that was estimated by way of the satellite signals and the true position at the place of the stationary receiver unit need not be the same as at the place of the vehicle. The greater the distance between the two is, the less suitable is the deviation measured by the stationary unit for improving the accuracy of the positioning of the vehicle or the more numerous must the stationary stations arranged along a road in order to make possible a precise positioning of the vehicles everywhere.